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Winter in the northern hemisphere is not quite over (the spring equinox is March 20), but it is already clear that, at least in the continental United States, there hasn’t been a winter like it since at least 1894.

The National Oceanic & Atmospheric Administration said Tuesday that the average temperature this season throughout the contiguous 48 states was 4.6 degrees Fahrenheit above average. The mean temperature on the mainland U.S. was 36.8 degrees Fahrenheit between December and February.

The previous record high winter temperature was set during 1999-2000.

Graphic courtesy NOAA

El Niño may have contributed, at least to some extent, to the unusually warm winter temperatures. The current ENSO is among strongest since at least 1950; a NOAA blog described it last summer as the “Bruce Lee of Niños.”

During an El Niño event the temperature of ocean water in the eastern Pacific rises beyond the normal range. The warmer ocean water off the west coast of South America drives changes in the jet stream, which in turn tends to cause winter temperatures in North America to be higher than they ordinarily would and increased precipitation in the southern portion of the continent. This explanation from a NOAA blog may be helpful:

Warmer tropical Pacific waters release more heat to the atmosphere, causing more rising air and storminess in the central and eastern tropics. The rising air moves north (and south) away from the tropics, traveling to the mid-latitudes, where it shifts the North Pacific jet stream farther southward and eastward. Movement and extension of the jet stream can bring more storms to the United States, and change the seasonal temperature and precipitation patterns.

This graphic shows typical ENSO warm episode impacts. Note that those impacts are not uniform throughout the world. Some areas can be warmer or cooler than others during this type of event. Graphic courtesy NOAA.

In fact, winter has been wet in North America this year. NOAA reported that, during the December-February period, the continental U.S. received the twelfth-most amount of precipitation for that time interval in recorded history.

The term El Niño is a Spanish phrase that means “little boy.” According to a University of Washington website:

The name El Niño (referring to the Christ child) was originally given by Peruvian fisherman to a warm current that appeared each year around Christmas. What we now call El Niño seemed to them like a stronger event of the same type, and the usage of the term changed to refer only to the irregular strong events. It wasn’t until the 1960s that it was widely realized that this was not just a local Peruvian occurrence, but was associated with changes over the entire tropical Pacific and beyond.

The opposite of El Niño is La Niña, which is marked by water temperatures in the eastern Pacific Ocean and near the equator that are below average.

Measurement of the extent to which ongoing anthropogenic climate change has driven the high temperatures experienced in the U.S. this winter is difficult. However, it is clear that ocean water temperatures are rising as greenhouse gases continue to be emitted to the atmosphere. This happens because, as the temperature of the air in Earth’s lower atmosphere rises, the oceans absorb some of the heat.

The oceans are now warmer than they have been in at least 50 years.

The surface of the world’s oceans has become warmer overall since 1880. In this graph, the shaded band shows the likely temperature range, which depends on the number of measurements and the methods used at different times. Source: U.S. Environmental Protection Agency, Climate Change Indicators (2014). Graphic courtesy U.S. Environmental Protection Agency.

The rising temperature of the oceans does likely cause warmer winter temperatures in at least some areas of the globe. When liquid water is heated, the molecules of the compound evaporate into a gas called water vapor. The process of evaporation adds heat to the surrounding atmosphere.

Residents of the western states and New England may face a relatively mild winter this year, according to a new prediction by the National Oceanic & Atmospheric Administration.

The agency’s U.S. Winter Outlook, released today, indicates that the coastal and inter-mountain west will likely experience average winter temperatures that are above normal.

“The temperature outlook for November-December-January (NDJ) 2014-15 indicates elevated probabilities of above-normal mean temperatures for Alaska, the far west, along the northern tier of the continental U.S. eastward to include parts of the northeast and mid-Atlantic,” the document said. “Below-normal mean temperatures are most likely over areas from eastern New Mexico to the western Gulf coast states.”

Graphic courtesy NOAA

Warmer does not mean drier, though. NOAA said that higher-than-average precipitation can be expected in southern California, the desert southwest, the southern Great Plains, across the deep south, and up the Atlantic coast to southern New England.

Graphic courtesy NOAA

That may be good news for water-starved California, though the drought there is not likely to end.

“While we’re predicting at least a two-in-three chance that winter precipitation will be near or above normal throughout the state, with such widespread, extreme deficits, recovery will be slow,” Mike Halpert, acting director of NOAA’s Climate Prediction Center, said.

An El Nino event could well happen this winter, but if it does, it is expected to be a weak one. NOAA estimated on Oct. 9 that there is a 67 percent chance of an El Nino event in the coming months.